Re: Promise SX8 performance issues and CARM_MAX_Q

[Jeff Garzik <jgarzik@pobox.com>]

Bernard Blackham wrote:
> Hi,
> 
> Playing with a recently acquired Promise SX8 card, we've found
> similar performance results to Matt's post to lkml a few months back
> at http://marc.theaimsgroup.com/?l=linux-kernel&m=110175890323356&w=2
> 
> It appears that the driver is only submitting one command at a time
> per port, which is at least one cause of the slowdowns. By raising
> CARM_MAX_Q from 1 to 3 in drivers/block/sx8.c (it was 3 in an
> earlier pre-merge incarnation of carmel.c), we're getting very
> notable speed improvements, with no side effects just yet.
> 
> Knowing very little about what this change has actually done, I've a
> few questions: 
> 
>  - Should this be considered dangerous?
>  - Why was it taken from 3 to 1?
>  - Is CARM_MAX_Q a number defined (or limited) by the hardware?

In multi-port stress tests, we couldn't get SX8 to function reliably 
without locking up or corrupting data, with more than one outstanding 
command.

Maybe a new firmware has solved this by now.

	Jeff